Lubricants encounter negative influences throughout their working life, including load-induced shear stress, thermal degradation, water contamination, aeration, wear metal catalyzing, and contamination from dirt, chemicals, and incompatible lubricants and fluids.
These external influences can produce an array of fluid property-altering effects that manifest as oxidation, polymerization, cracking hydrolysis, and fluid evaporation, which can lead to a major thickening or dilution in viscosity, acid buildup and reservoir sludge.
Lubricating oils consist of base oil and an engineered, consumable additive package designed for the oil’s intended service, such as gear oil, automotive engine oil, and air-tool oil. The additive’s function is to combat and stave off the effects of outside influences and deliver a reasonable oil life cycle, whatever ambient conditions prevail.
What About OEM Oil Change Interval Recommendations?
Because OEMs (original equipment manufacturers) have little or no control over the actual working and ambient conditions in which their systems operate, they make oil-change recommendations based on ideal conditions and a 40-hr. operating week.
Following only OEM recommendations could mean changing your oil too early or too late for your particular conditions.
That could translate as additional costs for meaningless oil changes or repairs. Thus, the only way to set an oil-change schedule is to regularly test and trend the lubricant to determine when and how it degrades.
Doing so can help an organization establish a fiscally responsible, condition-based approach toward oil-life-cycle management.
When determining an oil’s condition and remaining life expectancy, we must analyze its fluid properties to establish the presence of an additive package, its depletion rate, and its oxidization presence/effect.
To verify if and when an oil change is warranted, the lubricant is tested in numerous ways against a virgin oil baseline with two primary measured indicators leading the decision-making process:
1. Major changes (up or down) in an oil’s viscosity measure.
2. Increase in an oil’s acidity AN (Acid Number) measure.
Viscosity Testing
When looking at the Viscosity rating of new oil, typically, the viscosity is measured in Centistokes (cSt), the oil’s Kinematic Viscosity rating that depicts its measured resistance to flow and shear caused by the force of gravity.
As the oil thickens or dilutes over time, its specific gravity (SG) changes and can result in testing errors when using a gravity-based viscosity test.
A more consistent measurement is achieved by measuring its internal friction by checking for the Absolute Viscosity rating that depicts the oil’s resistance to flow and shear.
Since absolute viscosity is determined by multiplying kinematic viscosity by the actual specific gravity, it delivers an accurate and error-free trending measurement, making it the preferred choice for most oil labs. You will know the difference between the two viscosity scales, as absolute viscosity is measured in centipoise (cPs) instead of centistokes (cSt).
Due to the large variables encountered in oil use, working with a laboratory with experience setting up caution and critical limits for your industry type is preferred.
Typically most labs will start with a clearly defined set of viscosity working limits of -10% CL (Critical Lower), -5% CaL (Caution Lower), +5% CaU (Caution Upper), and +10% CU (Critical Upper) for Industrial oils. In more severe environments, the CaU and CU limits can be reduced to +4% and +8%, respectively. For oils with Viscosity Improvers, the lower limits are usually doubled.
Acidity Testing
The AN (Acid Number) measures the acid concentration in the oil—not the acid strength—and is greatly affected by the presence of water within the oil. Most virgin oil starts with an acid number less than 2.
Setting limits for oil acidity is not as easy as for viscosity, as caution and critical limits are set according to the type of additive package used in the oil. Most standard mineral oils are considered corrosive if measured above AN 4, whereas AW (Anti-Wear) or R & O (Rust and Oxidation) inhibited oils are considered critical well below AN 3.
Working with your oil supplier’s engineering department and a reputable oil lab with experience in your industry is the best way to set up meaningful, acceptable limits for your environment.
As in any trending analysis, the rate of change is arguably more important than the actual change number as it signifies a specific change event that has taken place that likely requires immediate investigation.
Originally published in The RAM Review.
